1. Field of the Invention
The present invention is directed to a method and apparatus for quickly heating a thermionic vacuum tube cathode thereby allowing use of the tube soon after it is switched on.
2. Description of the Prior Art
Most vacuum tubes use thermionic cathodes; i.e., cathodes comprising material which emits electrons when heated, thereby providing the electron beam used in the tube. Such tubes cannot be placed in useful operation until their cathodes are heated to a temperature sufficient to provide the necessary stream of electrons. It has long been an objective of manufacturers and users of vacuum tubes to minimize the length of time that it takes the cathode to heat up to its operating temperature.
Various methods and structures have been devised to meet the desire for a capability to quickly heat the cathode assembly used in vacuum tubes. One universally applied approach is to minimize the mass of the cathode structure. It is elementary that for a given thermal energy input, a cathode structure of lower mass will reach a given operating temperature faster than a more massive cathode structure of the same material. Reducing mass as a means to improve heat-up time is limited by the need for the cathode to contain a sufficient amount of thermoionic material to provide the desired electron current, along with the need for structural support which adds to the thermal mass of the cathode assembly.
Directly heated cathodes are heated by passing electrical current directly through the resistive body of the cathode, normally a wire. In such cathodes the rate of heating can be increased by initially increasing the current through the cathode beyond that necessary to maintain the cathode at its operating temperature. This approach is limited by the ability of the cathode to withstand higher current levels.
Indirectly heated cathodes have a separate heater element or filament placed in close proximity to the cathode, but electrically isolated therefrom. Heat is transferred from the heater to the cathode by radiation across a vacuum or by conduction through a thermally conductive, electrically insulative material in good thermal contact with both the heater and the cathode.
A heater need not be as massive as a cathode and therefore can be made to heat more rapidly. The rate at which heat is transferred from the heater to the cathode may be maximized by selecting materials of high emissivity and/or high thermal conductivity. Increasing the current through the heater during cathode warm-up, beyond the normal operating current, will cause the heater to heat more rapidly and thereby decrease the time needed to place the tube in operation. Again, this is limited by the ability of the heater materials to withstand the higher current and temperature, and the deleterious effects these increased factors have on the heater's useful life.
Indirect heating by conduction requires a very good thermal contact between the filament and cathode. The need to dispose electrically insulating material between the filament and the cathode adds to the thermal mass of the combined structure. Problems can arise due to thermal stress and cracking, resulting in degraded performance after a few warm-up cycles.
Another, somewhat different, approach allowing a vacuum tube to be placed in operation quickly is to maintain the cathode at or near its operating temperature at all times. While the related circuitry is off, the cathode heater is supplied with current to keep the cathode ready for operation. This approach permits almost instantaneous use of the tube when desired since there is no warm-up cycle. Nonetheless, maintaining the cathode in a heated state is costly in terms of energy usage, may be undesirable due to the fact that the apparatus is in an alive and heated state at all times, and will shorten the useful life of the tube.
Cathodes using impregnated tungsten or thoriated tungsten emitters are used in many high power microwave and power grid tube applications since they are capable of supplying the necessary high current densities over relatively long time periods. Such cathodes typically operate at higher temperatures than the more common oxide cathodes used in devices such as television cathode ray tubes. Therefore, in tubes using impregnated tungsten or thoriated tungsten cathodes, warm-up time can be a more significant problem due to the need to bring the cathode to a much higher temperature. Nonetheless, many of the applications for such tubes are very time-critical and the need for a very short warm-up cycle essential.
Accordingly, it is an object of this invention to provide a method and apparatus for quickly heating a vacuum tube cathode so that the tube may be placed in useful operation shortly after it is switched on.
It is a further object of this invention to overcome the limitations of prior art means for quickly heating a vacuum tube cathode, thereby decreasing the delay before a vacuum tube can be used.
Yet another object of this invention is to provide a quick-start method and apparatus useful with impregnated tungsten and thoriated tungsten cathodes.
Still another object of this invention is to provide a quick-start cathode assembly which allows the tube to be placed in use less than one second from the time it is switched on.